1. Field of the Invention
The present invention relates to a sheet conveying roller for conveying a sheet, a sheet conveying apparatus having the sheet conveying roller, and an image forming apparatus having the sheet conveying roller.
2. Description of the Related Art
Conventionally, a sheet conveying roller is provided in an image forming apparatus, a printer, a sheet processing apparatus for processing sheets, an image reading apparatus for reading originals, and a sheet handling apparatus, such as sheet conveying apparatus for conveying sheets, mounted in these apparatuses to convey sheets. The image forming apparatus specifically includes a printer, a copier, a facsimile, and a combined machine of them.
The sheet conveying roller includes a shaft supported by a bearing at both ends thereof, and a rubber roller as an elastic friction member mounted on outer circumference of the shaft, thus conveying the sheet by the rubber roller portion. The friction member may use a resin, which has equivalent friction factor with that of the rubber roller.
The shaft of the sheet conveying roller is a solid shaft finished by machining and polishing. The solid shaft is machined by lathe to give center drilling at the end thereof, cutting off and chamfering at each end thereof, and cutting step for a stepped shaft. Furthermore, the sold shaft is provided with a tightening groove or the like thereon by milling. In addition, the solid shaft is subjected to cutting to form a D-shaped cross section by a surface grinder, to drilling for arranging parallel pins by a drilling machine, and to polishing on the outer circumference by a polishing machine. Plating may further be applied in some cases. Thus machined solid shaft as a shaft single body is finished by directly baking rubber or by adhering a rubber roller thereon, followed by polishing the outer circumference of the rubber roller portion to obtain the sheet conveying roller.
The shaft of the sheet conveying roller uses a pipe shaft in some cases. The pipe shaft is prepared by introducing a hoop material of metallic thin belt of stainless steel, aluminum, iron, copper into a drawing die to curl the hoop material in the transversal direction thereof to form a cylindrical shape, followed by butting both ends of the curled portions to seamless-weld them into a shape of a pipe. The pipe shaft is cut to a specified length, as needed, to obtain a base pipe. The base pipe is drawn through a die into a specified outer shape. After correcting the bend of the pipe, the pipe shaft is treated by centerless polishing to a high precision outer diameter (Refer to Japanese Patent Application Laid-Open No. H11-165361).
The pipe shaft may be manufactured by what is called the drawing in which a metal is drawn by spinning, swaging, or pressing. The drawing allows the cylindrical pipe to vary the diameter along the longitudinal direction.
For using thus prepared pipe as the shaft of the sheet conveying roller, the pipe is cut to a specified length, and a stepped boss prepared by machining is integrally fixed to each end of the pipe by press-fitting, friction welding, caulking, or the like. Then, the pipe shaft is machined on the outer circumference thereof centering on the stepped boss to align the pipe shaft with the stepped boss, thus attaining a shaft free from an eccentric motion.
In some cases, a thick-walled pipe is subjected to high precision coaxial machining by lathing on the inner diameter at both ends, and an axial shaft with flange is fixed to each end thereof. Then, the polishing is given on the outer circumference of the pipe centering on the axial shaft at both ends to manufacture a stepped shaft with accurate alignment. The shaft is mounted in a mold of a rubber molding machine to bake a rubber to form a rubber roller thereon, followed by polishing the outer diameter of the rubber roller to assure accuracy of circle and alignment, thus obtaining the sheet conveying roller.
When the sheet conveying roller is used in a sheet conveying apparatus, a rubber roller is mounted at almost center of the shaft, or rubber rollers are arranged along the shaft at a certain interval, and, in some cases, further a bearing is mounted at each end of the shaft, while a gear and a pulley may be mounted at one end thereof. As described above, the conventional sheet conveying roller is manufactured by a plurality of machining processes. Accordingly, the conventional sheet conveying roller has problems of taking the large number of steps for manufacturing and of consuming machining time and cost.
The conventional sheet conveying roller also has problems that, in the step of baking the rubber, the heating of shaft takes a time to delay the molding cycle, which increases the molding cost and increases the energy consumption.
According to the conventional sheet conveying roller, if the shaft is a solid shaft, the mass is heavy and the inertia is large so that the starting time and stopping time of rotation take a long time.
Although the solid shaft has rigidity, the necessary rigidity is only a degree to attain the nipping and conveying force for a thin and light sheet. The conveying force, however, differs with the place of using the sheet conveying roller. For example, at the duplex sheet feeding portion and the retarding sheet feeding portion in a sheet feeder installed in an image forming apparatus, a separation pressure is required, while some of them require only a low separation pressure. The roller to feed a sheet from a cassette, or the registration roller to correct the skew feed of the sheet in the sheet feeder need a conveying force of a degree to override the stiffness of the sheet. On the other hand, the sheet conveying roller plays a role of bridging the sheet in the sheet conveying path, and the sheet conveying roller is subjected to relatively light load, thus the nipping pressure between the sheet conveying rollers is set to a low level in some cases.
Generally a hollow pipe is weaker in deflection than the solid pipe, and the hollow pipe is not welcomed to sheet feeding which requires reliability.
When the solid shaft is used in a sheet conveying roller, which requires only a relatively low strength, the diameter is reduced to save the material cost in some cases. Even if the shaft diameter is reduced, however, the friction roller is required to have a certain diameter. That is, in the sheet conveying section, to guide a curled sheet smoothly to a nip of the sheet conveying roller pair, there are required a certain gap in the sheet conveying path and a certain roller diameter. Those variables significantly affect the sheet conveying performance. When the diameter of the friction roller is reduced, the curled sheet becomes difficult to enter the nip of the sheet conveying roller pair, which causes the skew feed and jamming in the sheet conveying path.
Consequently, even when the outer diameter of the solid shaft is reduced, the outer diameter of the friction roller has to be increased to some extent. As a result, the material cost of the friction roller increases to increase the total cost. To this point, the conventional sheet conveying roller is not provided with the rubber roller thereon over a wide range in the longitudinal direction of the shaft for the case of being mounted in a sheet conveying path, but is provided with the rubber rollers in parts of the shaft at an interval along the longitudinal direction of the shaft to save the material cost of the rubber roller.
Regarding the functions practically required for the sheet conveying roller, the shaft is requested to rotate the rubber roller, to be supported at both ends thereof, and to hold gear, pulley, and the like in the drive transmission mechanism. Similarly, the rubber roller is requested to keep the friction force to surely handle the sheet, and even under abrasion, to keep the sheet conveying speed and friction force within a certain abrasion range. The shaft is not necessarily a solid shaft, and the rubber roller as the friction member does not need a large thickness.
Accordingly, there has been waited a lightweight sheet conveying shaft in which the shaft is a pipe shaft, and the thickness of the friction member more than it seems to be necessary is not required.